1. Field of the Invention
This invention relates to vehicular automatic transmission systems providing a plurality of gear reduction ratios, such as automatic mechanical transmissions (AMT's), and to control systems therefor. In particular, the present invention relates to control systems and methods for vehicles with throttle controlled engines and automatic transmissions wherein gear selection and shift decisions are made and executed based upon measured and/or calculated parameters such as transmission output shaft or vehicle speed, transmission input shaft and/or engine speed, throttle position, calculated engine speed in a potentially engagable ratio, and the like. More particularly, the present invention relates to automatic transmission control systems of the type described above having a selectable mode of operation in which downshifting, by more than a single ratio step if required, to the lowest allowable speed will be commanded and vehicle deceleration is sensed and the lowest allowable ratio is calculated in view of the sensed deceleration.
2. Description of the Prior Art
The use of automatic transmissions of both the automatic mechanical type utilizing positive clutches and of the planetary gear type utilizing frictional clutches is well known in the prior art as are control systems therefor. Electronic control systems for automatic transmissions wherein gear selection and shift decisions are made based upon certain measured and/or calculated parameters such as vehicle speed, engine speed, rate of change of vehicle speed, rate of change of engine speed, throttle position, rate of change of throttle position, full depression of the throttle (i.e. "kickdown"), actuation of the braking mechanism, currently engaged gear ratio, and the like are known in the prior art. Examples of such automatic transmission control systems for vehicles may be seen by reference to U.S. Pat. Nos. 4,595,986; 4,361,060; 4,073,203; 4,253,348; 4,038,889; 4,226,295; 3,776,048; 4,028,929; 4,039,061; 3,974,720 and 3,942,393, all hereby incorporated by reference. Semi-automatic transmission systems wherein operator selected shifts, including skip-shifts, are, if allowable, automatically executed are known in the prior art as may be seen by reference to U.S. Pat. No. 4,648,290, the disclosure of which is hereby incorporated by reference.
Automated transmission systems having a selectable mode wherein skip downshifts, if allowable (i.e. engine speed in to be engaged ratio not greater than maximum allowable engine speed), assuming substantially constant vehicle speed, are known in the prior art as may be seen by reference to U.S. Pat. No. 4,576,065, the disclosure of which is hereby incorporated by reference.
While the above referenced automatic transmission control systems, and similar systems, are effective to control an automatic transmission by selecting a desired gear ratio which will tend to optimize the fuel economy and/or performance of the vehicle in view of the sensed parameters and then to command or allow a shift into the selected gear ratio, such control systems were not totally acceptable as the predetermined logic rules utilized to generate shift decision date such as a shift pattern or patterns (also commonly referred to as a shift point profile) or the like did not optimize vehicle performance when commanding a downshift by selecting the lowest allowable ratio assuming constant vehicle deceleration. In particular, the relatively large inertia of a heavy vehicle ascending a relatively steep off-road grade (for example, a grade of more than twenty percent (20%)) would tend to cause the vehicle to decelerate quickly and result in multiple rapid single downshifts which may be objectionable and cause further vehicle deceleration and/or stalling of the engine due to lack of power supplied to the vehicle from the engine during the out of gear portion of the shift transient.
While the system disclosed in above-mentioned U.S. Pat. No. 4,576,065 did have a mode of operation forcing allowable skip downshifts, the system did not evaluate expected vehicle deceleration and thus certain desirable/allowable skip downshifts were not performed.